This invention relates to a period measurement system for measuring the period of a biosignal, particularly of a signal representative of the heartbeat of a fetus.
A conventional system for measuring the period of a biosignal relies upon a correlation system adapted to derive an autocorrelation function of the biosignal, and to measure the period of the biosignal of the basis of the autocorrelation function.
The period measurement system that relies upon the correlation system operates by sampling a biosignal over a suitable sampling period, computing the autocorrelation function of the biosignal from the sampled data, and detecting the peaks of the biosignal from the computed autocorrelation to thereby obtain the period.
The autocorrelation function indicates the similarity between two portions of the biosignal wave form at two different times separated by a certain time interval. In other words, it represents the degree of similarity of the repeating biosignal waveform. This can be better understood from FIG. 1, wherein it is seen that if a portion M.sub.1 which repeats at a certain period T is shifted along the time axis by an interval of time which is equal to the period T, the portion M.sub.1 will be superimposed on the immediately succeeding portion M.sub.2 with maximum accuracy.
In order to obtain the autocorrelation function from the biosignal, we may write the autocorrelation function A(.tau.) in terms of the biosignal f(t) which is a function of the time t. Thus, A(T) may be written ##EQU1## in which T represents the period of the biosignal and .tau. represents a time interval between two points in time separated by a given interval, the earlier point in time being a reference time in connection with the biosignal. In other words, .tau. is a variable which applies a phase difference to the biosignal f(t) along the time axis.
Reference will now be had to FIG. 2 to describe the conventional period measurement system that relies upon the correlation function to measure the period of a biosignal, specifically a signal representative of the heartbeat of a fetus, which signal will be referred to as a "heartbeat signal" hereafter.
In FIG. 2, a probe 2 is brought into contact with, say, the abdomen of a female subject to extract the fetal heartbeat signal for the purpose of measurement. The heartbeat signal so detected has its waveform suitably processed in a preprocessing circuit 3 and then sampled at a predetermined sampling period in a sampling circuit 4. The data obtained by sampling the heartbeat signal is stored in a data memory 6 composed of a plurality of shift registers. As each item of new data enters the data memory 6, items of data already stored up to that point are shifted to the immediately adjacent register, so that data is shifted sequentially from one register to another, with the oldest item of data in the last register being lost as each new input arrives. A multiplier 8 and an adder 10 constitute an autocorrelation function computing circuit which is adapted to compute an autocorrelation function using the data stored in the data memory 6. A correlation memory 12 stores the results of the computation, namely the computed autocorrelation function. Thus the autocorrelation function is computed by the multiplier 8 and the adder 10 on the basis of the data stored in the data memory 6. The computation is performed on the basis of single sampling-cycle divisions and, for each item of data X.sub.1, X.sub.2, X.sub.3 . . . , proceeds in the manner X.sub.1 .multidot.X.sub.s+1 +A.sub.1 .fwdarw.A.sub.1, X.sub.1 .multidot.X.sub.s+2 +A.sub.2 .fwdarw.A.sub.2, . . . , X.sub.1 .multidot.X.sub.s+m +A.sub.m .fwdarw.A.sub.m, the result of each computation being stored sequentially in the correlation memory 12. By repeating these computation and storage operations for n cycles, data defining the autocorrelation function is stored in the correlation memory 12. Peaks representing the periodicity of the autocorrelation function stored in the correlation memory 12 are detected by a peak detector 14 in order to obtain the period of the biosignal.
In the conventional measurement system of the type described, however, the arrangement is such that the phase difference variable .tau. is varied in each single sampling cycle. It is therefore necessary to store in the correlation memory 12 the results of each and every autocorrelation function computation covering the entire body of data spanning the range over which the variable .tau. is varied in each sampling cycle. This means that the correlation memory must have a very large storage capacity. In addition, even when measuring a signal having a short period the computations described above are performed over a time interval corresponding to from two to three times the length of the period, so that much of this computation is without substantial meaning. This fact also calls for a correlation memory of a large storage capacity and is also disadvantageous when viewed in terms of real-time processing owing to the fact that a large number of substantially meaningless computations are performed.